Indirect ophthalmoscopy techniques are used in diagnostic, therapeutic and surgical procedures in the field of ophthalmology, and normally include the use of positive power lens systems in conjunction with an observing optical system such as a indirect ophthalmoscope, slit lamp biomicroscope, or operating microscope. Lenses conventionally used in indirect ophthalmoscopy applications often perform the dual functions of condensing light from a light source toward the pupil of the eye to illuminate the fundus, and forming an inverted real aerial image of the fundus which can be viewed with either a monocular or binocular device. Indirect ophthalmoscopy systems have been found to be superior to other examination or treatment methods, in particular in the examination of a retinopathies, retinal separation, retinal tumors, intraocular foreign bodies, and provide the ability to see fundus lesions which otherwise may not be observable when opacities of the ocular media are present. Hand-held lenses used in indirect ophthalmoscopy have been of a variety of types, each affording various advantages in the examination of the fundus. In the development of indirect ophthalmoscopy, hand-held lenses originally comprised a single lens element having plane and/or convex spherical surfaces and were of low power. The aerial image produced with such a lens was magnified and inverted, and considerably blurred, particularly toward the periphery of the formed aerial image. A two-element Ramsden style indirect ophthalmoscopy lens was thereafter produced by Rodenstock utilizing a spherical convex surface on each of the lens elements. This lens provided improved optical quality over that of a single lens using spherical surfaces, but resulted in increased surface reflections and light loss which outweighed the limited benefits of the insufficiently improved image quality. There has also been reported the use of a "periscopic lens" for use in slit lamp funduscopy. The periscopic lens included a double plano-convex lens system similar to the Ramsden style indirect ophthalmoscopy lens, but of smaller diameter and higher power. The periscopic lens system also utilized spherical surfaces, such that only limited improvement in optical quality was realized and a reduced field of view was obtained.
Subsequently, improvements were noted by the use of slightly higher powered single element lenses, each having one aspheric surface with the other surface being plano or spherical. Although the use of a single aspherical surface in the indirect ophthalmoscopy lens did show great improvement over spherical indirect ophthalmoscopy lenses, lens aberrations remained. Thus, the formed aerial image of the fundus still had aberrations and increasing astigmatic effects particularly in the peripheral regions of the image.
These designs were subsequently improved upon by the use of two aspherical surfaces incorporated into the indirect ophthalmoscopy lens. The first use of a double aspheric indirect ophthalmoscopy lens designed for use with an indirect ophthalmoscope was described in U.S. Pat. No. 4,738,521 by David Volk. This lens design incorporates both front and back aspheric surfaces of revolution of conoid type, which substantially improved the quality of the formed aerial image by reducing aberrations including field curvature, astigmatism and distortion. The use of double aspheric lenses has been found to be a distinct advantage in indirect ophthalmoscopy and has made possible the use of much stronger lenses while providing increased clarity of the image as well as increased field of view. More recently, a symmetrical double aspherical indirect ophthalmoscopy lens particularly suited for use with a slit lamp biomicroscope was developed by David Volk, and described in U.S. Pat. No. 4,627,694. The double aspheric lenses shown in this patent were of smaller diameter and higher power, with the aspheric surfaces described as providing improved correction of aberrations, including field curvature, astigmatism, and distortion. Lenses made according to this design have demonstrated themselves to be better suited for use with the slit lamp biomicroscope, and have yielded significant improvement in the examiner's ability to see details in the aerial image of the fundus. The single element double aspheric lens thus provided improved imaging as well as wide field viewing of the fundus, particularly for use with a slit lamp biomicroscope.
In the development of indirect ophthalmoscopy lens systems, it is seen that particular lenses are best for use with a slit lamp or other biomicroscope, while other lens systems are more suited for use with an indirect ophthalmoscope or other observation system. For example, the longer focal length, lower power indirect ophthalmoscopy lens systems are not suitable for use with a slit lamp biomicroscope, as the observation of the formed aerial image would require positioning of the biomicroscope at a location which exceeds the range of travel built into the instrument. The development of the double aspheric indirect ophthalmoscopy lens as described above has enabled use of higher power lenses to provide a shorter working distance within the range of adjustment of the slit lamp biomicroscope to facilitate its use as the observation optical system, instead of an indirect ophthalmoscope or the like. It thus may be seen that a particular diagnostic or therapeutic procedure may require the use of a lens system which is particularly designed for that procedure only, thus requiring the practitioner to purchase and maintain a variety of lens systems on hand to achieve the best results for other particular procedures. Based upon the foregoing, it is clearly desirable to provide the practitioner an indirect ophthalmoscopy lens system which may be modified in a specific and pre-determined manner such that a variety of examination or treatment procedures are possible.
Similar in this respect to indirect ophthalmoscopy lens systems, other ophthalmic lens systems have inherent limitations as it relates to examination or treatment procedures. For example, direct ophthalmoscopic observation techniques utilize a direct ophthalmoscopy lens system which produce a virtual image of the eye fundus having particular magnification characteristics. With both such direct and indirect ophthalmoscopic lens systems, there exists no ability to modify the inherent magnification, field, or imaging characteristics of the system, thereby limiting their use. Similarly, gonioscopic lenses for examination and treatment of the anterior chamber angle of the eye are also limited in optical performance, having specific magnification and field characteristics. In these various lens systems for examination and treatment of the eye, no provision exists for the modification of inherent imaging characteristics of the lens itself and thus the resulting field characteristics and magnification of the produced virtual or aerial image. Only with the slit lamp biomicroscope or operating microscope does there exist the ability to change magnification, not of the viewed image itself, but as a secondary adjustment of the Keplerian telescope observation system.